Page 1 of 1

κDCX cable fault and hi-pass problem

PostPosted: March 27th, 2019, 4:00 am
by franjam
Hi Ti,
hope all is well at AMB ; been offline for a year or so, (enjoying my kDCX ;) ), but am back with a fault that seems complicated to solve, at least for me.

Before coming to that, my kDCX has also developed an annoying intermittent shutdown problem which I have concluded comes from the cable connecting the PSU to the DSP PCB. If I poke the cable with my finger it all comes back, however I can't close the case because it invariably comes back :x . I have resoldered the connections once and it worked fine for a while before becoming faulty again ; I can only conclude that there is a break in the copper within the cable. I believe all kDCX builders should replace this crappy cable by default as I am not alone with this problem. That said, could you help with identifying the parts needed, on Farnell for example ? The choices are bewildering for the unitiated.

Ok, here is the problem which has occurred recently, ( kDCX rock-solid before this :) ) : I decided , to complete and finish off my system, to build two Elliot Sound Product P3A amplifiers from scratch, one after the other. ( these were an affordable and technically accessible choice for me ). These were to replace my two rebuilt A&R A60 power amps and render the sytem more visually pleasing with the two power amps in one box and two PSU's also in another seperate box. My KDCX is set up for a two-way sytem with the crossover at 2000Hz. The first P3A amp worked fine and I used it on the tweeters while I worked on the rest. I then installed the 2nd P3A on the second heatsink in the case and that's when the problems started:
The 1st amp, which had been working fine up until this point ,started to overheat on one channel quite alarmingly, (output transistors) and the power fuses (3.15A) on the +/- rails both ended up blowing. The sound from the tweeters was completely aneamic and upping the gain on the hi-pass would not solve the problem.
I thought the problem might be oscillation as the amp case is not very big and all the connections are fairly close together by necessity. So I took the "faulty" amp out and left the 2nd P3A in on its own. ( the 'faulty" amp susequently tested fine on the basic tests). I re-connected an A60 on the hi-pass channel and low and behold same problem!!! :x . Hardly any sound from both tweeters and the output transistors on one channel overheating ! If I persist or turn the sound up the speaker fuses blow. Both amps were working fine before being connected to the hi-pass (outputs 2&4), and both work fine on the low-pass.
I checked the Psu's , one toroidal was giving 36v on the - rail and 34v on the + rail, so I swapped it out for another I had, this has not solved the problem.

By deduction the problem does not seem to come from the amps or the PSU's although why a fault would suddenly occur in kDCX at the same time is beyond me. I have gone back and done all the basic tests stipulated at the end of the kDCX build and all seems well so I am really quite lost as to where to go next. What externally could cause the output transistors to overheat? Why the aneamic sound from the tweeters?
At the moment I am using one P3A amp with the fullranges and no Xover on outputs 1&3, all is well but this is a big comedown in sound quality and no long -term solution...any help would be appreciated,

Many thanks,

franjam

P.S. I have considered trying ouputs 5&6 for the hi-pass but this obviously does not cure the problem.

kDCX cable fault and hi-pass problem

PostPosted: March 27th, 2019, 8:34 pm
by amb
Regarding the power cable from the DCX2496’s PSU to the DSP board, some versions had a connector at the DSP board that seem to be problematic. If yours has such a connector, you can try resoldering the PCB joints to see if it makes any difference. I think later DCX2496s eliminated the connector and has directly soldered wires.

As for your other troubles, have you tried re-assigning the roles of the DCX2496’s output channels to see if they all exhibit the same behavior? How did you establish that the problem is within the DCX2496 rather than at the power amp?

It seems to me that perhaps you are encountering an oscillation problem with your power amp(s). Maybe due to ground issues. Also, do all power amp outputs have reasonable DC offsets?

Are all amps balanced? If not, are you using any XLR-RCA adapters or cables?

Do you have other (known working) power amps to test with, just to experiment and isolate the problem?

Re: κDCX cable fault and hi-pass problem

PostPosted: March 28th, 2019, 4:11 am
by franjam
Hi Ti
Regarding the power cable from the DCX2496’s PSU to the DSP board, some versions had a connector at the DSP board that seem to be problematic. If yours has such a connector, you can try resoldering the PCB joints to see if it makes any difference. I think later DCX2496s eliminated the connector and has directly soldered wires.


As I said i already resoldered and it seemed to work for a while but the problem came back. I think I'm going to do a point to point direct solder using IDE cable, what do you think?

As for your other troubles, have you tried re-assigning the roles of the DCX2496’s output channels to see if they all exhibit the same behavior? How did you establish that the problem is within the DCX2496 rather than at the power amp?


Not yet , I will take the time to try... same reaction with 2 power amps made me think it was the DCX..

It seems to me that perhaps you are encountering an oscillation problem with your power amp(s). Maybe due to ground issues. Also, do all power amp outputs have reasonable DC offsets?


Yes it does seem like oscillation I am awaitig fuses to retest the amps but I have definitely lost one channel on one of the amps unfortunately.

Are all amps balanced? If not, are you using any XLR-RCA adapters or cables?


Amps are not balanced and I am using XLR-RCA with pins 2/3 on the negative.

Do you have other (known working) power amps to test with, just to experiment and isolate the problem?


I'm working on it:-).

The thing is that the problem started when I added the two P3A amps in the same smallish case, probably too close together, when I tried to put things back as they were the problem has persisted, go figure...